It is imperative that sources of renewable energy be coupled with energy storage media for efficient and instant energy supplementation to the electrical grid. Large-scale stationary batteries offer several advantages over competing technologies (e.g. pumped hydro, compressed air storage, flywheels, and redox flow batteries) such as small environmental impact, no moving parts (i.e. low maintenance costs) and potentially very long lifetimes. To date, much research has focused on Li-based grid-scale battery technology which suffers from several setbacks, most notably cost and safety; its high energy density better suits Li technologies for transportation and portable applications. Long cycle lifetimes will be required for stationary grid energy storage and membrane crossover (redox flow batteries) or electrode degradation (Li-ion and lead-acid batteries) lead to capacity loss over time despite intensive research. Moreover, there is a general need for new, alternative battery technologies and Zn is emerging as a favored energy carrier in recently developed systems.
Molten salt-based energy storage, especially its sub-class of liquid metal batteries (LMBs), is one viable option for electrochemical grid storage due to its low cost, high charge carrier concentration, and predicted very long cycle lifetime (1000+ cycles) (10, 11). LMBs employ “structureless” electrodes which cannot physically degrade and reduce the possibility of dendrite formation that plagues solid Li and Zn metal-containing batteries. “Liquid metal battery” has previously referred to a three-layer design (anode-electrolyte-cathode) achieved using materials with different densities at the operating temperature; however, here we expand the definition to include any battery which employs liquid metallic electrodes (in any orientation). Emerging LMB technologies have thus far been limited to materials (electrode and electrolyte) demonstrating this self-separating layered design which is attractive from a fabrication standpoint (i.e. at minimum, only a simple vessel with a metallic base contact is required) but the stratified design is susceptible to electrode dissolution or magnetohydrodynamic instabilities at high current densities which could lead to cell shorting and thermal runaway. Furthermore, this specialized cell design (requiring immiscible materials with the density of molten salt between that of the anode and cathode) precludes many potential active and low-cost materials, and often requires exceedingly high operating temperatures (>500° C.). Therefore, orientation of electrodes in the non-traditional configuration introduced in this work, where an arc is the shortest path between anode and cathode rather than a line, opens the door for a plethora of high temperature battery materials to be investigated.
An ultimately commercially valuable large scale LMB should have moderate operating temperatures, economical components, and long term cycling stability. Up to now, much research has foregone these marketable features, instead focusing on cells with reactive and costly components that achieve high power output or high nominal voltage. ZEBRA, Mg—Sb, and Ca-based batteries exemplify this point. ZEBRA, while capable of a moderate working temperature (245° C.) is limited in robustness with its beta alumina solid electrolyte and in safety with liquid Na and [AlCl4]− active components. Mg—Sb and Ca-based LMBs rely on reactive metals and must operate at exceedingly high temperatures (500-700° C.) in order to liquefy the materials. In the present work, we seek to address the shortcomings of these battery technologies while keeping material/energy costs low. Favorable properties of the above have been combined with Zn-based alloys and the low melting ZnCl2:KCl eutectic to demonstrate a LMB which is promising for large scale energy storage.